Process for the production of synthetic plastics comprising urethane groups and biuret groups

ABSTRACT

A process is provided for the production of synthetic polyurethane plastics, particularly foam plastics, containing biuret groups by reacting polyisocyanates, polyhydroxyl and/or polycarboxyl compounds, flame inhibitors and, if desired, blowing agents, emulsifiers, activators and other additives which is characterized by using as the polyisocyanate solutions in monomeric polyisocyanates of from about 1 per cent to 85 per cent of polyisocyanates containing biuret groups and having the formula: ##EQU1## in which R is an aromatic, araliphatic or cycloaliphatic radical which may, if desired, be substituted and 
     n is an integer from 0 to 5, wherein the proportion of biuret-polyisocyanates having more than three isocyanate groups is at least 20 per cent by weight based on the total quantity of biuret-polyisocyanates, and the solutions contain from about 0.03 per cent to about 5 per cent by weight of chemically combined emulsifiers.

This is a division of application Ser. No. 360,883, filed May 16, 1973,issued as U.S. Pat. No. 3,899,454, which application is acontinuation-in-part of application Ser. No. 264,641 filed June 20, 1972now abandoned which application is a continuation of application Ser.No. 036,500 filed May 11, 1970, now abandoned.

Polyurethane plastics, particularly polyurethane foams, have a varietyof physical properties and are obtained by the isocyanate polyadditionprocess from compounds having several active hydrogen atoms andpolyisocyanates, activators, stabilizers and other additives, if desiredwith concurrent use of water and/or other blowing agents. By thisprocedure it is possible to choose components which will yield elasticand also rigid foam plastics or other plastics having properties whichlie between these limits.

It is also known to produce synthetic plastics, particularly foamplastics, which contain urethane groups and biuret groups. For example,according to the disclosures of U.S. Pat. Nos. 3,124,605, 3,392,183,3,441,588, 3,232,973, 3,284,479, 3,367,956 and of U.S. patentapplication Ser. Nos. 551,916 and 760,085, a wide variety of lowmolecular weight polyisocyanates containing biuret groups and theirunpurified solutions in monomeric polyisocyanates can be used inaccordance with processes known per se for the production ofpolyurethane synthetic cellular and noncellular plastics. However, thefoam plastics containing biuret groups produced by these processes havenot been industrially exploitable because of their low compressivestrength and heat resistance.

It is therefore an object of this invention to provide polyurethaneplastics containing biuret groups and a process for preparing them whichare devoid of the foregoing disadvantages.

Another object of this invention is to provide polyurethane plasticscontaining biuret groups which have a high compressive strength and heatresistance.

Yet another object of this invention is to provide polyurethane foamscontaining biuret groups which have extremely low weights per unitvolume, freedom from shrinkage, excellent powers of insulation and aparticularly regular pore structure.

A further object of this invention is to provide a process for theproduction of polyurethane containing biuret groups wherein thecomponents can be mixed homogeneously and easily within a short time topreclude premature and undesired reaction.

A still further object of this invention is to provide a process for thepreparation of foamed polyurethane plastics containing biuret groupswhich allows a much more highly uniform distribution of the blowingagent during the foaming operation so that a more regular blowing actionand a greater uniformity of foaming can be achieved to yield a finer,more regular pore structure.

An additional object of this invention is to provide a process by whichelastic and rigid polyurethane plastics containing biuret groups can beproduced as well as polyurethanes having properties which lie betweenthese limits.

The foregoing objects and others which will become apparent from thefollowing description are accomplished in accordance with thisinvention, generally speaking, by providing polyurethane plasticscontaining biuret groups which have substantially increased compressivestrengths and heat resistance and which are prepared by reacting organicpolyhydroxyl and/or polycarboxyl compounds with a solution in monomericpolyisocyanates of from about 1 to about 85 per cent of a polyisocyanatecontaining biuret groups and having the formula ##EQU2## in which R isan aromatic, araliphatic or cycloaliphatic radical which may, ifdesired, be substituted and

n is an integer of from 0 to 5, wherein the proportion ofbiuretpolyisocyanate having more than three isocyanate groups is atleast 20 per cent and preferably 40 to 70 per cent by weight based onthe total quantity of biuret-polyisocyanate, and the solutions containfrom about 0.03 per cent to about 5 per cent and preferably 0.1 to 2 percent by weight of chemically combined emulsifiers. If desired blowingagents may be included in the reaction mixture if a cellular structureis desired and/or inhibitors, activators, emulsifiers and otheradditives may also be added.

By the practice of this invention it is now possible to prepare foamshaving improved compressive strength and heat resistance compared toproducts produced from monomeric polyisocyanates and biurettriisocyanates. The properties of the foams produced by the process ofthis invention are improved to such an extent that they may be usedindustrially although they can be prepared from biuret polyisocyanateproducts obtained by processes known per se at high temperatures and inthe presence of emulsifiers which react with isocyanates. Such biuretpolyisocyanates contain a certain proportion of higher molecular weightbiuret polyisocyanate having an increased functionality and chemicallybonded emulsifiers. Apart from the foregoing advantages it has beenfound that such biuret polyisocyanates offer other important advantagesin the formation of foamed plastics, particularly in the production ofsemi-hard and hard foams. For example, when producing hard foams by theprocess of this invention it is possible to prepare cellular plasticswhich have a lower weight per unit volume together with increased heatresistance, a particularly regular pore structure, freedom fromshrinkage, improved compressive strength and excellent powers ofinsulation. It would appear that the progress of the expanding andsetting reactions in the formation of the foam is at least partlyresponsible for this highly advantageous combination of properties sincea controlling factor is the heretofore unknown uniformity and speed withwhich carbon dioxide is released when using water as a blowing agent inthe practice of this invention, even when other blowing agents such astrichloromonofluoromethane are also used.

Various advantages are also provided in the actual process for theproduction of foam plastics. When using only monomeric liquidpolyisocyanates, the mixing of the foam-forming components with thepolyhydroxyl and the like compounds, which are always of high viscosity,is frequently difficult and often requires relatively long time periodsin order to achieve the necessary homogeneity. As a consequence,premature and undesired reactions which have an undesirable effect onthe final product can take place. The biuret-polyisocyanate systemscontaining chemically combined emulsifiers used in the process of thisinvention are conveniently and surprisingly highly effective solutionpromoters and can be mixed easily in the shortest possible time with theother components of the reaction mixture to preclude undesired prematurereactions which would otherwise take place in a non-homogeneous reactionmixture. It was also surprising to find that the setting and expandingtimes during the foaming process in the practice of this invention aremore favorable compared to the setting and expanding times of normalcommercial systems, thus yielding a greater uniformity of foaming.

An additional advantage of the process of this invention is that thewater used as blowing agent is distributed much more uniformly duringthe foaming operation and a more regular blowing action is obtained sothat foam plastics having a finer, more regular pore structure areproduced.

The biuret-polyisocyanate systems containing chemically combinedemulsifiers to be used in the practice of this invention can be preparedin a manner known per se by any of the processes described, for example,in U.S. Pat. Nos. 3,124,605, 3,232,973, 3,441,588, 3,383,400, 3,284,479,3,367,956, 3,392,183, 3,350,438 and in U.S. patent application Ser. Nos.551,916 and 760,085 from any choser polyisocyanates. In one preferredembodiment, the biuret-polyisocyanate mixtures having a relatively highNCO functionality are advantageously prepared by reacting the water orcompound which splits off water with the polyisocyanate startingmaterial in the presence of from about 0.03 to about 5 per cent byweight, and preferably from about 0.1 to about 2 per cent by weight, ofemulsifiers containing OH, amino, amido, --COOH, --

or urethane groups reactive with NCO groups. The emulsifier which isoperative even at a very low concentration in a very large excess ofmonomeric polyisocyanates thus reacts with the isocyanate and istransformed into an emulsifier containing isocyanate groups. It isimmaterial whether the emulsifier is introduced with water, for example,or whether is added to the monomeric polyisocyanate starting materialbefore the formation of biuret groups begins. By choosing suitableconcentration ratios of the reactants to the monomeric isocyanates andby controlling the temperature, it is possible to obtain clear mixtureswhich are stable on storage and which have increased NCO functionality.

The content of biuret polyisocyanates having a functionality of four tosix or higher can easily be increased by continuously introducing smallquantities of water into an approximately 50 per cent solution of biurettriisocyanates in monomeric diisocyanates such as tolylenediisocyanates, for example. Because of their high concentration, thealready formed biuret triisocyanates largely react further to formpolyisocyanates which are completely soluble in the mixture at roomtemperature and which have a functionality of four to six or higher.

The proportion of higher molecular weight biuret polyisocyanates canalso be increased simply by heating to temperatures from about 160° toabout 180°C. The proportion by weight of higher molecular weight biuretpolyisocyanate can be established by fractional precipitation andchromatographic analysis.

For the production of the polyisocyanate mixtures of this invention, thebiuret polyisocyanates are dissolved in liquid monomericpolyisocyanates. It is preferred to use unpurified solutions of thebiuret polyisocyanates in monomeric polyisocyanates and to actuallyprepare the required concentration of the polyisocyanate mixture whenproducing the biuret polyisocyanates. This can be done, for example, bypreparing highly concentrated solutions which, if required, are dilutedwith the various types of monomeric polyisocyanates and further dilutedto the required biuret polyisocyanate content.

If desired, however, the polyfunctional polybiuretpolyisocyanates canalso be prepared by heating a,w-diisocyanates containing one or moreurea groups in excess monomeric polyisocyanates or polyisocyanatemixtures to temperatures in the region of 180°C. in a first step.Solution occurs and reaction to form polybiuret-polyisocyanates ofhigher molecular weight takes place. Such polybiuret-polyisocyanates canbe used for the production of the foam materials of this invention incombination with various types of di- and polyisocyanato urethanes,which can be obtained by reacting an excess of a diisocyanate with lowmolecular weight triols or diols and, if desired, with solublepolyisocyanates containing isocyanurate groups. Mixtures of suchpolybiuret-polyisocyanates which have been oxidized and modified at hightemperature by the action of oxygen with the formation of hydroperoxidegroups can also be used, as well as those polybiuret-polyisocyanatesobtained by gassing polyisocyanates with air or oxygen in the presenceof tin tetrachloride, iron chloride, copper and zinc complexes ofSchiffs bases from stearylamine and salicylaldehyde.

Biuret-polyisocyanate solutions which are particularly importanttechnically and, hence, preferred in the practice of this invention forproducing synthetic polyurethane plastics, particularly foam plastics,are biuret-polyisocyanate mixtures such as those obtained by the actionof e.g. water on aromatic, araliphatic, cycloaliphatic and aliphaticpolyisocyanates such as, for example, 1-methylbenzene-2,4-diisocyanate,1-methylbenzene-2,6-diisocyanate as well as their technical mixtures,arylene diisocyanates and their alkylation products, m- and p-phenylenediisocyanates, naphthylene diisocyanates, diphenylmethane diisocyanates,di- and triisopropylbenzene diisocyanates, triphenylmethanetriisocyanates, thiophosphoric acid tri-(p-isocyanatophenyl)-esters,phosphoric acid tri-p-isocyanatophenyl)-esters, aralkyl diisocyanatessuch as 1(isocyanatophenyl)-ethyl isocyanate, m- and p-xylylenediisocyanate, alkylene diisocyanate, alkylene diisocyanates such astetra- and hexamethylene diisocyanates, trimethylhexamethylenediisocyanate, isophorone diisocyanate,1-methylcyclohexyl-2,4-diisocyanate and isomers thereof,a,w-diisocyanatocarboxylic acid esters, polyisocyanates substituted byany different substituents such as, for example, alkoxy, nitro, chlorineor bromine groups. Addition products of polyisocyanates with less thanequivalent quantities of polyhydroxyl compounds, such as trimethylolpropane, hexanetriol, glycerine, butane diol and the like as well aspolymerization products of the aforesaid polyisocyanates with severalisocyanurate rings per molecule, if desired, addition products of two toeight mols of any polyisocyanate with one mol of an aldimine or ketimineand polyisocyanates which are obtainable by aniline-formaldehydecondensation and subsequent phosgenation are suitable for the productionof modified biuret types.

Higher molecular weight polyisocyanates which can be produced byreacting monomeric polyisocyanates with higher molecular weightcompounds containing hydrogen atoms reactive with NCO groups includinghigher molecular weight polyhydroxyl compounds, polycarboxyl compoundsand polyamino compounds can also be used. Mixtures of differentpolyisocyanates can also be used for forming biuret groups.Diphenylmethane diisocyanates containing carbodiimide groups oruretonimine groups as described, for example, in German Pat. No.1,092,007 and 4,4'-diphenylmethane diisocyanate modified by less thanequivalent quantities of dipropylene glycol and tripropylene glycolwhich are liquid and technical isomers thereof are also eminentlysuitable for the production of biuret-polyisocyanates. Polyisocyanatescontaining semicarbazide groups and biuret groups produced as described,for example, in U.S. patent application Ser. No. 760,085, fromunsymmetrically disubstituted hydrazines, for example, are alsosuitable. These biuret-polyisocyanates are valuable antioxidants andcolor stabilizers under heat and aging conditions for polyurethane foamplastics and synthetic plastics. Modified polyisocyanates obtained bythe telomerization of ethylenically unsaturated compounds withpolyisocyanates are also very suitable for the production of thebiuretpolyisocyanates to be used in the practice of this invention.Biuret-polyisocyanate mixtures obtainable by hydrogenation polyadditionreactions such as those which are prepared by hydrogenating polynitrocompounds and polynitriles in the presence of excess quantities ofmonomeric polyisocyanates, for example, are likewise suitable.

Monomeric polyisocyanates which are eminently suitable for theproduction of biuret-polyisocyanates are1-methylbenzene-2,4-diisocyanate, 1-methylbenzene-2,6-diisocyanate andits technical isomer mixtures such as, for example, mixtures of 80 to 65parts by weight of 1-methylbenzene-2,4-diisocyanate and 20 to 35 partsby weight of 1-methylbenzene-2,6-diisocyanate. When usingbiuret-polyisocyanates prepared from tolylene diisocyanates, in theprocess of this invention for the production of foam materials it isparticularly surprising that by choosing suitable polyhydroxyl and/orpolycarboxyl compounds, hard foam plastics having improved heatstability and a more uniform pore structure can be obtained compared tosimilarly produced foam plastics prepared from the same polyols andpolyisocyanates obtained by the phosgenation of aniline-formaldehydecondensates. However, 4,4'-diisocyanatodiphenylmethane, its technicalisomers and the technical crude distillates of these mixtures are alsoexcellent for use in the formation of readily soluble, liquidbiuretpolyisocyanates of relatively high molecular weight.

Excellent reactive emulsifiers having at least one hydrogen atomreactive with NCO groups to be used in the production of thebiuretpolyisocyanate systems of this invention include, for example,hydroxyl containing oleyl alcohol polyglycol ethers, castor oilpolyglycol ethers, isononylphenol polyglycol ethers,3-benzyl-4-hydroxy-diphenyl polyglycol ethers, higher molecular weightpolyglycol ethers having an average molecular weight of about 2000 withan n-dodecyl group and a terminal OH group, emulsifiers containing aminoterminal groups corresponding to the foregoing examples with amineterminal groups such as those which can be obtained by cyanoethylationand subsequent hydrogenation of the aforesaid emulisifiers. The variousammonium salts of oleic acids and long-chain fatty acids and their saltswith primary and secondary amines are likewise eminently suitable sincethey dissociate in the polyisocyanate starting material and the aminesreact as ureas while the longer chain carboxylic acids are transformedinto isocyanato-acyl amide derivatives.

The higher molecular weight biuret-polyisocyanate mixtures of thisinvention in dissolved form are generally solid or resin-like compoundsor more or less viscous oils at room temperature. They have asurprisingly high solubility in monomeric liquid diisocyanates andpolyisocyanates such as tolylene diisocyanates or polynuclear liquid ormelted polyisocyanates such as 4,4'-diisocyanatodiphenyl methane, itstechnical isomers and liquid modifying agents which contain carbodiimidegroups as well as dicyclohexylmethane-4,4'-diisocyanate,diphenyldimethylmethane-4,4'-diisocyanate, 4,6-dimethyl-1,3-xylylenediisocyanate, 2,6-diethylbenzene-1,4-diisocyanate, m-and p-xylylenedissocyanates, isophorone diisocyanate, hexamethylene diisocyanate,tetramethylene diisocyanate and the like and mixtures thereof. Moreover,the biuret-polyisocyanates of this invention have excellentcompatibility and miscibility with adducts of any chosen polyisocyanateand low molecular weight polyols such as, for example, trimethylolpropane and glycerine, said adducts comprising different types ofisocyanate groups, and they are also miscible with polymerized lowmolecular weight polyisocyanates which may contain m- and severalisocyanurate rings in the molecule, if desired.

The biuret-polyisocyanate solution in monomeric polyisocyanates used inthe practice of this invention are liquid, clear, storable mixtureshaving preferably a viscosity of between about 10 and about 100,000 andmost preferably 20 to 10,000 centipoises measured at 21°C., for example,in the case of the preferred polyisocyanates which are based on tolylenediisocyanate or on 4,4'-diisocyanatodiphenyl methane, with 45 to 50 percent by weight solutions. Their mixtures with one another and also inliquid industrial polyisocyanates which are obtained by the phosgenationof aniline-formaldehyde condensates are also of a relatively lowviscosity. When using 65 to 70 per cent by weight solutions, it is alsoadvantageous that relatively low viscosities of from about 1400 to about2000 centipoises, measured at the same temperature are found.

The biuret-polyisocyanate solutions used in the process of thisinvention are employed in varying quantities depending upon whetherrigid or elastic foam plastics are to be produced. Generally, theproportion of liquid monomeric polyisocyanate is kept low when highlycrosslinked, rigid foam plastics are required while, on the other hand,more monomeric polyisocyanate is used as a solvent when the aim is toproduce elastic type products. In each case, it is desirable to adjustthe polyisocyanate mixtures so that the solutions are liquid at roomtemperature. Generally speaking, the quantities of biuret-polyisocyanatemixtures introduced amount to from about 1 to about 85 per cent byweight based on the total weight of the isocyanate solution.

Any suitable compounds containing hydroxyl and/or carboxyl groups may bereacted with the polyisocyanate solutions of this invention, especiallywhen foam plastics are to be produced. Polyhydroxyl compounds areusually those having molecular weights of from about 500 to about 3500including, for example, polyesters, polyethers, polythioethers,polyacetals, polycarbonates, polyester amides and the like and mixturesthereof containing several hydroxyl groups such as those which are knownper se for the production of homogeneous or cellular polyurethanes aslisted, for example in U.S. Pat. Nos. 3,201,372 and 3,275,674.

Some particular hydroxyl polyesters which may be used include, forexample, reaction products of polyhydric alcohols with polyvalentcarboxylic acids such as those which are used in many applications inindustry. Instead of the free carboxylic acids, it is also possible touse the corresponding anhydrides or esters of the polycarboxylic acidsor mixtures of these compounds for the production of the hydroxylpolyesters. The polycarboxylic acids can be aliphatic, cycloaliphatic,aromatic and heterocyclic compounds which may be substituted and/orunsaturated, if desired. Some such suitable acids include succinic acid,adipic acid, sebacic acid, phthalic acid, isophthalic acid, phthalicanhydride, maleic acid, maleic anhydride, monomeric, dimeric andtrimeric fatty acids, dimethyl terephthalate and the like and mixturesthereof. Some specific examples of polyols which can be used includeethylene glycol, propylene-1,3-glycol, butylene-1,4-glycol,butylene-2,3-glycol, glycerine, trimethylol propane, hexane-1,2,6-triol,butane-1,2,4-triol, trimethylol ethane, pentaerythritol, mannitol,sorbitol, methyl glycoside, polyethylene, polypropylene and polybutyleneglycols and the like and mixtures thereof. Polyesters having terminalcarboxyl groups are likewise suitable for reaction with thepolyisocyanates in accordance with the invention.

The hydroxyl polyethers suitable for use in the practice of thisinvention are also those of the type known per se and include, forexample, the polymerization products of epoxides such as ethylene oxide,propylene oxide, butylene oxide, styrene oxide or epichlorohydrin withstarting materials having reactive hydrogen atoms such as, for example,alcohols or amines including glycerine, trimethylol propane, ethyleneglycol, ammonia, ethanolamine, ethylene diamine and the like andmixtures thereof. Sucrose polyethers can also be used in the practice ofthis invention.

Some further suitable organic polyhydroxyl compounds which may also beused in the practice of this invention are described, for example, in"Polyurethanes, Chemistry and Technology", Volumes I and II, Saundersand Frisch, Interscience Publishers, 1962 and 1964 (page 32f, Volume Iand page 5 and page 198f, Volume II) and also in the KunststoffHandbuch, Vol. VII, Vieweg-Hochtlen, Carl-Hancer Verlag, Munich, 1966,for example on pages 45 to 71. Epoxy resins, hydrogenation products ofethylene-olefine-carbon monoxide copolymers, phenolformaldehyde,ureaformaldehyde resins reacted with alkylene oxides and the like canalso be used. Low molecular weight polyhydroxyl compounds of the typealready mentioned, for example, and/or chain lengthening agents such asglycols, diamines, water, aldimines, ketimines and the like can beconcurrently used in portions.

The production of the foam plastic itself is effected by known processesat room or elevated temperature simply by mixing the polyisoyanatemixtures with the carriers of the hydroxyl and/or carboxyl groups, andwater, accelerators, emulsifiers and other auxiliaries such asflame-inhibiting substances and blowing agents. It is advantageous forthis purpose to use machines such as those described, for example, inU.S. Reis. No. 24,514.

Any of the large number of suitable flame-inhibiting substances known inthe prior art which contain phosphorus and halogen atoms may be added tothe reaction mixture. Antimony, bismuth or boron compounds are alsoknown for this purpose. A review of known and useful flame inhibitorswhich may be employed is given in the chapter "FlammhemmendeSubstanzen", pages 110-111 in Kunststoff Handbuch, Vol. 7,"Polyurethanes", by Vieweg-Hochtlen, Carl-Hanser Verlag, Munich, 1966.The flame inhibitors are usually employed in quantities of from about 1to about 20 per cent by weight, and advantageously from about 1 to about15 per cent by weight, based on the quantity of polyisocyanate mixturebeing used.

Any desired blowing agents may be employed including, for example,alkanes, haloalkanes, low-boiling solvents in general and the like suchas, e.g., methylene chloride, monofluorotrichloromethane,difluorodichloromethane, acetone, methylformate and any of those listedin U.S. Pat. No. 3,201,372. Compounds releasing gas at a relatively hightemperature such as azo compounds or diurethanes of bis-semiacetalsformed from 2 mols of formaldehyde and 1 mol of ethylene glycol can alsobe used as blowing agents.

Any desired urethane activators can be used including, for example,tertiary amines, such as triethylamine, dimethylbenzylamine,tetramethylethylene diamine, n-alkyl morpholines, endoethylenepiperazine, urotropine, hexahydrotriazines, such astrimethylhexahydrotriazine, 2,4,6-dimethylaminomethyl phenol or organicmetal salts, such as tin-(II)-acylates, e.g. tin-(II)-salts of2-ethylcaproic acid, dialkyltin-(IV)-acylates, such asdibutyl-tin-dilaurate or acetyl acetonates of heavy metals, e.g. of ironand the like including those catalysts listed in U.S. Pat. No.3,201,372.

Any desired emulsifiers may be used including ethoxylated phenols,higher sulphonic acid, sulphonated castor oil, ethoxylated castor oil,sulphonated ricinoleic acid, ammonium salts of oleic acid and the like.Any desired foam stabilizers can be used including, for example, thosewhich are based on polysiloxane-polyalkylene glycol copolymers or basicsilicone oils. Other emulsifiers, catalysts and additives which can beused are, for example, listed in "Polyurethanes, Chemistry andTechnology", Vols. I and II, Saunders and Frisch, IntersciencePublishers, 1962 and 1964.

The quantities of polyisocyanate solutions containingbiuretpolyisocyanates which are used should generally be sufficient sothat the number of NCO groups is at least equivalent to the sum ofreactive hydrogen atoms which are present although, if required, the NCOgroups can also be used in excess or in a less than an equivalentquantity. When foam plastics are being produced and water is used as ablowing agent, an excess of polyisocyanate sufficient to react with thewater to blow the foam is used. Excess quantities of isocyanates can beincorporated into the foam plastic during the foaming process by addingtrivalent or pentavalent phosphorus compounds such as phospholidines;phospholine oxides, tertiary esters, amides or ester amides ofphosphorus or phosphoric acid as isocyanurate groups, uretdione groupsand/or carbodiimide groups.

The foam plastics which are obtained by the process of this inventioncan be widely used, for example, in the building industry as buildingboards, sandwich elements, ceilings, partitions; for heat insulation inrefrigerators, cold-storage plants, refrigerated trucks and containers;in road and rail construction for technical insulation of pipes, for theinsulation of tank storage installations and in ship construction as airfilters and filters for hydrocarbons in internal combustion engines aswell as for packaging material for protection against impacts. The foamplastic products obtained by the process of this invention can be hard,semi-hard or flexible, so that they can also be used as upholsterymaterial. It is also possible to employ the process of this invention tomanufacture semi-hard and mold-foamed polyurethane plastics having acompact surface and a cellular core. Surprisingly in such a process,smooth, homogeneous and heat-resistant marginal zones and cellular coresare obtained, so that the improved heat-resistance and compressivestrength of these products provide a considerable technical advance, forexample, in the insulation of pipe conduits for superheated steam.

The invention is further illustrated but is not intended to be limitedby the following examples in which all parts and percentages are byweight unless otherwise specified.

EXAMPLE 1

A. Production of Biuret-Polyisocyanate Solution in MonomericPolyisocyanate

About 1218 parts of 1-methylbenzene-2,4-diisocyanate in which about 1,5parts of a castor oil polyethylene glycol ether (OH-number 40) have beendissolved, are heated to about 98°C. About 27 parts of water areconstantly introduced dropwise over a period of about 21/2 hours withgood stirring. On completion of the addition of water, the temperatureis raised to about 165°C. for about 45 minutes. Cooling then takes placeand a solution of starting material is obtained which is approximately67 per cent biuret-polyisocyanate mixture and which has a viscosity of1580 centipoises at 21°C. The NCO content of the solution is 34.6 percent.

For analytical examination, this solution is freed from monomeric1-methylbenzene-2,4-diisocyanate at about 135°C/0.2 mm Hg by thin-flimevaporation. The NCO content of the biuret-polyisocyanate resin is 24.8per cent. About 100 parts of the resin are dissolved in 300 parts byweight of a mixture of ethyl acetate/acetone (1:1) and divided intoeight fractions by adding each time 120 parts of cyclohexane andremoving the precipitates formed.

Each fraction has an NCO content of from about 24.3 per cent to about25.3 per cent. However, after the fractions have been converted intopolyurethanes using ethanol as a monoalcohol, they differ in theirosmotically measured molecular weight. Thereafter, about 34 per cent ofthe biuret-polyisocyanate resins consist of tetraisocyanates andpentaisocyanates and about 65 per cent by weight of triisocyanate;namely N,N',N"-tri-(3-isocyanato-4-methylphenyl)-biuret, the triethylurethane of which has a molecular weight of about 660 (calculated 634).

B. Production of a Polyester Foam Plastic From the Solution ofBiuret-Polyisocyanate Starting Material A

About 80 parts of a polyester obtained from 5.1 mols of adipic acid, 1mol of phthalic anhydride and 8.4 mols of hexane triol and having anhydroxyl content of 8.5 per cent are mixed with about 20 parts of apolyester obtained from 1.43 mols of adipic acid, 1 mol of hexane trioland 1 mol of butylene-1,3-glycol with an hydroxyl content of 6.5 percent and thoroughly stirred together with an activator mixture of about2 parts of an ester obtained from 2 mols of diethyl ethanolamine and 1mol of adipic acid as well as 5 parts of about a 54 per cent aqueoussolution of the sodium salt of castor oil sulphate. While stirring well,about 107 parts of the solution described above under A, which has anNCO content of 34.6 per cent, is added. Uniform foaming takes place anda hard foam plastic containing urethane and biuret groups is obtainedwhich is characterized by the following properties:Weight per unitvolume kg/m³ 34Compressive strength (kg/cm²) 2.4Heat stability (°C) 164

The modified polyisocyanate mixture can be very easily incorporated intothe above polyester mixture and pore formation is uniform.

When compared against biuret-triisocyanate solutions in excess tolylenediisocyanate as described in Example 1 of British Pat. No. 889,050, thephysical properties of the foam plastic obtained using the process ofthis invention show a substantial improvement in compressive strengthand heat stability. For example, in Example 1 of British Pat. No.889,050, 503.7 parts of 2,4-tolylene diisocyanate (strength 99.4 percent) were stirred under nitrogen while a solution of 5.4 parts of waterin 80 parts of acetone was added at 35° to 37°C. during one hour. Theslightly exothermic reaction was controlled by occasional externalcooling. The thick suspension was stirred at 35° to 37°C. for a furtherthree hours, then raised to 155° to 160°C. during about one hour, whileremoving acetone by distillation. The reaction mixture was maintained at155° to 160°C. during another two hours. The resulting approximately 30per cent solution of triisocyanato-N,N',N"-trisubstituted biuret inexcess tolylene diisocyanate was a clear mobile liquid containing 40.7per cent isocyanate groups. When 90 parts of this product was stirredwell with the polyol-mixture described above under A, a hard foamplastic is obtained which is characterized by the followingproperties:Weight per unit volume kg/m³ 32Compressive strength (kg/cm²)1.5Heat stability (°C.) 148

EXAMPLE 2

A. Production of Biuret-Polyisocyanate Solution in MonomericPolyisocyanates

About 1050 parts of a technical isomer mixture of tolylene diisocyanatecontaining about 80 per cent of 1-methylbenzene-2,4-diisocyanate andabout 20 per cent of 1-methylbenzene-2,6-diisocyanate into which hadbeen previously stirred about 2 parts of a castor oil polyethyleneglycol ether (OH-number 40). About 20.8 g. of water are then introduceddropwise at about 60°C. over about 30 minutes with stirring and theprecipitate which forms is dissolved during subsequent heating at about170°C. After maintaining the temperature for about 3 hours at about170°C., the solution is quickly cooled to room temperature. The productthus obtained has an NCO content of 33.7 per cent and a viscosity(25°C.) of 3000 cP.

B. Production of a Polyether Hard Foam Plastic

About 100 parts of a sorbitol polyether having an OH number of 480 andobtained by reacting sorbitol with propylene oxide are mixed with about40 parts of trichloromonofluoromethane, about 1 part of a siliconestabilizer, about 1 part of endoethylene piperazine and the mixture isthoroughly stirred with about 107 parts of the biuret-polyisocyanate ofpart A with an NCO content of 33.7 per cent. Uniform foaming takes placeand a hard foam plastic containing urethane and biuret groups isobtained which has the following properties:

    Weight per unit volume (kg/cc)                                                                        30                                                    Compressive strength (kg/cm.sup.2)                                                                    3.3                                                   Heat stability (°C.)                                                                           147                                               

The foam plastic is dimensionally stable for more than three hours at -30°C. and more than five hours at +100°C.

EXAMPLE 3

A. Production of a Biuret-Polyisocyanate Solution

About 150 parts of a technical isomer mixture of about 80 per cent1-methylbenzene-2,4-diisocyanate and about 20 per cent1-methylbenzene-2,6-diisocyanate into which about 2 parts of a castoroil polyethylene glycolether (OH-number 40) had been stirred previouslyare treated with about 36.8 parts of water which is introduced dropwiseat about 60°C over about 30 minutes with stirring. The mixture is thenheated to about 170°C, which temperature is maintained for about threehours. To the reaction product thus obtained is added an equal quantityby weight of a phosgenated aniline-formaldehyde condensate, containingabout 50 per cent of 4,4'-diisocyanato diphenylmethane and about 50 percent of polymeric MDI.

B. Production of Hard Polyether Foam

About 108 parts of the polyisocyanate mixture prepared as described inExample 3A are foamed as described in Example 2B with about 90 parts ofa sucrose polyether obtained by reacting sucrose with propylene oxideand having an OH number of 380, about 10 parts of N,N-dihydroxyethylamino methane phosphonic acid diethyl ester, about 10 parts oftrichloroethyl phosphate, about 40 parts of trichloromonofluoromethane,about 1 part of endoethylene piperazine and about 1 part of a siliconestabilizer (Sf 1109 of General Electric).

The hard foam thus obtained has the following properties:

    Weight per unit volume (kg/cc)                                                                        30                                                    Compressive strength (kg/cm.sup.2)                                                                    2.5                                                   Heat stability (°C.)                                                                           117                                               

The foam plastic was self-extinguishing according to ASTM/D 1692-67 T.

EXAMPLE 4

A. About 1000 parts of pure 4,4-diisocyanato diphenylmethane in which 5parts of castor oil polyethylene glycolether (OH-number 40) aredissolved are reacted with 16,2 parts of water like described in Example1 A. A liquid biuret polyisocyanate is obtained which has an NCO-contentof 22,8 per cent.

B. About 100 parts of a sorbitol polyether having an OH-number of 480and obtained by reacting sorbitol with propylene oxide are mixed withabout 1 part of silicon stabilizer (Sf 1109 of General Electric), about40 parts of monofluortrichloromethane, about 1 part of endoethylenepiperazine and the mixture is thoroughly stirred with 160 parts of thebiuretpolyisocyanate of Example 4A.

The hard foam plastic thereby obtained has the following properties:

    Weight per unit volume (kg/cc)                                                                        38                                                    Compressive strength (kg/cm.sup.2)                                                                    4.3                                                   Heat stability (°C.)                                                                           134                                               

The foam plastic was dimensionally stable for more than 3 hours at -30°and more than five hours at +100°C.

It is to be understood that any of the components and conditionsmentioned as suitable herein can be substituted for its counterpart inthe foregoing examples and that although the invention has beendescribed in considerable detail in the foregoing, such detail is solelyfor the purpose of illustration. Variations can be made in the inventionby those skilled in the art without departing from the spirit and scopeof the invention except as is set forth in the claims.

What is claimed is:
 1. A polyisocyanate which comprises a solution in amonomeric polyisocyanate of from about 1 to about 85 percent of a biuretpolyisocyanate having the formula: ##EQU3## in which R is an aromatic,araliphatic or cycloaliphatic radical and wherein n is an integer offrom 0 to 5 and the proportion of biuret-polyisocyanate having more thanthree isocyanate groups is at least 20 percent by weight based on thetotal quantity of biuret-polyisocyanates and the solution contains fromabout 0.3 to about 5 percent by weight based on the weight of the biuretpolyisocyanate of an emulsifier chemically bound to the polyisocyanate,the biuret polyisocyanate being prepared in the presence of theemulsifier, said emulsifier being selected from the group consisting ofhydroxyl containing oleyl alcohol polyglycol ether, castor oilpolyglycol ether, isononyl phenol polyglycol ether, higher molecularweight n-dodecyl polyglycol ethers having a terminal OH group and thecorresponding amine derivatives obtained by cyanoethylation andsubsequent hydrogenation by the foregoing compounds.
 2. Thepolyisocyanate of claim 1 wherein the solution of the biuretpolyisocyanate contains organic polyisocyanates containing urethanegroups.
 3. The polyisocyanate of claim 1 wherein the solution of thebiuret polyisocyanate contains organic polyisocyanates containingisocyanurate groups.
 4. The polyisocyanate of claim 1 wherein theproportion of the biuret polyisocyanate having more than threeisocyanate groups is from about 40 to about 70 percent by weight basedon the total quantity of biuret polyisocyanate.
 5. The polyisocyanate ofclaim 1 wherein the solution contains from about 0.1 to about 2 percentby weight of chemically combined emulsifiers.
 6. The polyisocyanate ofclaim 1 wherein the emulsifiers are chemically bound to the isocyanatethrough an OH, amino, amido, --COOH, --SH or urethane group reactivewith NCO groups.
 7. The polyisocyanate of claim 1 wherein the emulsifieris an hydroxyl containing oleyl alcohol polyglycol ether, castor oilpolyglycol ether, isononylphenol polyglycol ether, 3-benzyl-4-hydroxydiphenyl polyglycol ether, polyglycol ethers having an average molecularweight of about 2000 which contain an n-dodecyl group and a terminal OHgroup; the corresponding amine derivatives of the foregoing compounds;the ammonium salts of oleic acids and long chain fatty acids or theirsalts with primary or secondary amines.